The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells as a specific example of the prior art.
Solar cells are devices that convert the sun's energy into electricity using the photovoltaic effect. Solar power is an attractive energy source because it is sustainable and non-polluting. Accordingly, a great deal of research is currently being devoted to developing solar cells with enhanced efficiency while maintaining low material and manufacturing costs.
Most common solar cells are those based on silicon, more particularly, a p-n junction made from silicon by applying a n-type diffusion on a p-type silicon substrate, coupled with two electrical contact layers or electrodes. In order to minimize reflection of the sunlight by the solar cell, an antireflection coating, such as silicon nitride, is applied to the n-type diffusion layer. Using a silver paste, for example, a grid-like metal contact may be screen printed onto the antireflection layer to serve as a front electrode. This electrical contact layer on the face or front of the cell, where light enters, is typically present in a grid pattern made of “finger lines” and “bus bars”. Finally, a rear contact is applied to the substrate, such as by applying a backside silver or silver/aluminum paste followed by an aluminum paste to the entire backside of the substrate. The device is then fired at a high temperature to convert the metal pastes to metal electrodes. A description of a typical solar cell and the fabrication method thereof may be found, for example, in European Patent Application Publication No. 1 713 093.
For improving efficiency solar cells have been developed not only comprising antireflective dielectric front side layer but also a dielectric layer on their backside. By electrical passivation of the surface of a solar cell recombination of charge carriers is reduced, which has a positive effect on the efficiency of a solar cell. Most effective solar cells may be produced if formation of recombination centers is avoided during metallization, i.e. applying a selective emitter, reducing the metalized area and contacting the solar cell only along the contact fingers. The passivated area e.g. under the busbars and/or solder pads is not affected by the metallization.
After applying a metal containing composition, firing of the layered substrate the solar cells are interconnected to modules by soldering of solder ribbons.
In WO 2011/066300 A1 a so-called PERC (passivated emitter and rear contact) silicon solar cell and a process for preparing the cell is described. The backside electrode is produced by applying and drying a silver paste pattern on a perforated dielectric passivation layer on the backside of a silicon wafer. The silver covers only a part of the wafer's back surface, i.e. bare areas are left onto which an aluminum paste for formation of an aluminum back electrode is applied. The silver paste is not especially defined but shall have a “poor” fire-through capability and comprises particulate silver and an organic vehicle. Typical thick film conductive compositions have high metal powder contents, i.e. silver powder contents, of about 80 wt. % or even more.